As a country again stared deep into its soul, Sterling and Castile’s relatives were among the most prominent, searing voices. “I, for one, will not rest or not allow him to be swept in the dirt,” said Quinyetta McMillon, 31, the mother of Sterling’s 15-year-old son, Cameron, who sobbed as she addressed a press conference.

A video appeared to show two officers approaching Sterling, 37, with one tackling him over the hood of a car, then both apparently restraining him on the pavement. “As this video has been shared across the world,” McMillon said, “you will see with your own eyes how he was handled unjustly and killed without regard for the lives that he helped raise.

“As a mother, I have now been forced to raise a son who is going to remember what happened to his father that I can’t take away from him. He is at an age of understanding. I hurt more for him and his loss.”

Castile, a 32-year-old cafe supervisor at a Montessori school, had an equally articulate champion in his girlfriend Diamond Reynolds, who livestreamed the aftermath of his shooting to a worldwide audience. When asked about Thursday’s massacre of police, she replied: “This thing that has happened in Dallas, it was not because of something that transpired in Minnesota today.

“This is bigger than Philando,” she said, adding the names of other black men and women who have died in police custody. “This is bigger than all of us.”

Valerie Castile, center, is kissed by her son’s girlfriend, Diamond Reynolds, on Thursday as they march in Minnesota. Photograph: Jeff Wheeler/AP

Castile’s mother, Valerie, showing remarkable poise in a CNN interview, said starkly: “I think he was just black in the wrong place.”

The authenticity of these voices rang clear and true through the noise of America’s 24-hour news cycle, rightwing radio shock jocks, lurid commentary on social media and the interventions of Hillary Clinton, Donald Trump and other politicians. It illustrated how the families of the victims of police shootings have emerged as key figures in the national effort to comprehend events.

'This is bigger than Philando': family waits to speak to investigators about shooting

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Among those watching the week’s tragedies unfold was Sandy Phillips, whose daughter, Jessica, was among 12 people killed when a gunman opened fire in a crowded cinema in Aurora, Colorado, in 2012. “These people are in such shock that it pains me to see them interviewed so early. But I know it’s exactly what America needs to see and what America needs to hear,” she said on Friday. “People say they’re dancing on the graves of the dead but it’s the NRA [National Rifle Association] that’s dancing on the graves of the dead.”

Phillips and other affected parents have taken an increasingly significant role in the broader campaign against gun violence, seeking to prevent deaths at the hands of police, criminals, terrorists or mentally disturbed individuals. They lobby politicians, campaign on social media, run support groups and give speeches at candlelight vigils, Capitol Hill and the White House.

But though they speak with the unimpeachable authority of experience, they are not necessarily heeded by the influential gun lobby. Phillips, who channelled her grief into Jessi’s Message, an advocacy group for victims and survivors, said: “To see people who have lost loved ones – this is not OK, this is not my America any more. The NRA still try to demonise them and make them unimportant, but these men and women are a force. They won’t give up.”

When Barack Obama unveiled new executive actions on gun control in January, he was introduced by Mark Barden, whose seven-year-old son Daniel was murdered during the 2012 shooting at Sandy Hook elementary school in Newtown, Connecticut. Barden, who had been a professional musician, chose to make it his life’s work to save other families from the same pain.

“I found I had no choice,” explained Barden, a managing director of Sandy Hook Promise, a gun violence prevention organisation. “I had been handed this mission and I made a conscious decision to throw myself into it 100%.”

Po Murray, chair of another organisation, the Newtown Action Alliance, agreed: “We’re seeing more and more families jumping into the gun violence prevention arena because they don’t want to see another family go through what they went through. We love them: they are true heroes of the fight. As the number of gun violence victims keeps growing, there is no way the NRA can win.”

In the case of Castile, his girlfriend Diamond Reynolds was not only eulogist but journalist. She used her phone to post extraordinary footage to Facebook Live that appeared to show the moments after police shot her boyfriend, who can be seen slumped in the car, his shirt soaked in blood. She can be heard telling an officer: “You shot four bullets into him, sir. He was just getting his license and registration, sir.” While this took remarkable presence of mind, Reynolds’ shock and fear are also palpable.

Phillips, of Jessi’s Message, compared Reynolds with the mother of Emmett Till, a 14-year-old African American murdered after whistling at a white woman in 1955, who insisted that her son be displayed in a glass-topped casket so the world could see his beaten body. “Philando Castile’s girlfriend is so poised and collected to have committed to live video,” she said. “She can’t have realised how badly he was shot.

“This appears to be a shooting that didn’t have to happen. If there had been a video of the Aurora theater or Newton school shootings, this would have been stopped. We’ve now had several where it’s up close and personal. This is how we are getting our news: right through our phones, immediate and tragic. God willing, it will change the way we react to gun violence in this country.”